Grain and cereal harvest are hampered internationally whenever damp fall weather prevents crops from naturally drying in the field. Threshing cannot be completed because there is no large scale viable mechanical process to satisfactorily and economically dry grains and cereals. As a result, yields and product quality seriously deteriorate while farmers wait for favourable sun and wind conditions to naturally dry the kernels. And, if these conditions to not occur, crops can be lost entirely.
Accordingly, there is a long-felt need for means to safely dry these grains and cereals in a temperature controlled environment in order to preserve their commercial value.
Industrial processing of a wide variety of materials produces fine waste by-products which must be disposed of. Many are slurries of fine organic or inorganic particles suspended in water, and are referred to in industry as "sludge". Others are fibrous or chemically contaminated natural and artificial materials of varying consistency. Rigid and increasingly stringent environmental standards and legislation very tightly control disposal of these waste products. Sources vary widely and include pulp and paper mills, sewage treatment plants, large dairy farms, potash mines, coal mines, oil sand plants, chemical plants, wineries, dry cleaning plants and many other processing operations.
A typical sludge or slurry consists of 20% to 30% organic and/or inorganic solids and the balance is water. Handling this material is difficult because of the high water content, but also because it frequently contains chemicals or heavy materials which are harmful to the environment, or biologically active components which are dangerous to humans.
In the past, in order to reduce handling and disposal costs, industry's focus has been on devising methods to concentrate these waste materials by reducing the water content. This is accomplished mechanically by using equipment like belt presses or centrifuges in the processing stream, (common in the pulp and paper industry) or by constructing expensive and large holding ponds (common in sewage treatment and mining operations) where the material is allowed over time to settle and naturally concentrate. These methods achieve a maximum concentration of about 40% solids, but do not remove the harmful chemicals and metals, or sterilize the active biological elements.
Recently, industry has been searching for more effective means to "dry" industrial waste and at the same time environmental agencies have introduced legislation forcing mechanical treatment and more secure handling and disposal of such waste. Several systems have been designed for this purpose. These systems use indirect heating methods and consist of kilns, furnaces, burners and a variety of continuous and batch feed ovens. Typically, the heat energy in these systems is transferred to the material being processed by blowing hot air across the material, or by directing the material over hot heating surfaces. In the process, large volumes of air must be used and this air becomes contaminated by contact with the waste product as a result of picking up small quantities of fine particles, as well as by capturing volatile gases released by the material as it dries. As a result, this "contaminated" air requires processing before being released into the atmosphere. Such systems tend to be large, expensive and not portable, and furthermore produce a dried end-product that has been burned and therefore is of limited use for recycling.
One recent solution to the drying of sewage sludge is found in PCT application number PCT/CA90/00074 of Schmidt et al, published Sep. 7, 1990. This application describes a proposed mobile method and apparatus for drying sewage sludge in which the sludge is conveyed on tiered helical conveyors through a heated chamber and is subjected to radiant heat. The radiant heat is indicated as being supplied by a plurality of identical burner chambers disposed side by side. Each of the burner chambers provides an equal amount of heat. Air is heated in the burners and blown through the hollow axles of the auger shafts and through holes in the shafts to mix with the sludge. As the sludge dries, it is described as releasing steam which is drawn off into a condenser where the water and hot vent gases are separated. The vent gases are recycled through the burner chambers, where harmful gases are broken down.
At the exit of the heated chamber, it is suggested sewage sludge will typically have been reduced to a maximum solids content of from 80% to 95%. The sewage sludge is input to the helical conveyors through an open supply funnel and a helical feeder conveyor.
This design has several problems. Firstly, there is no continuous supply mechanism. During normal operation, supply of sewage sludge to the helical conveyors can be disrupted and result in an irregular supply of sewage to the helical conveyors. Irregular supply could damage components of the dryer since the extreme heat produced by the burners would not be mitigated by the heat sink effect of the drying sewage sludge. Burning of the sludge as well as serious plugging could occur.
Also this prior art dryer has no apparent means to control heat distribution at machine start up, therefore subjecting all internal components to very damaging high heat stress which dramatically affects useful machine operating life.
Further, this prior art dryer does not distribute flue gases in a manner that would follow the heat transfer gradient, which declines along the drying path. That is, as sewage sludge travels through the machine water is gradually lost through evaporation which significantly reduces the sludge's ability to absorb heat, yet this design provides equal heat energy throughout the machine, making no provisions for the diminishing heat gradient.
Further, the helical conveyors described in this prior disclosure render it difficult to move sewage sludge along the conveyor, and the individual transfer chutes located at the end of each auger flight may be subject to plugging.